Part of the upgrade process happens in the first "terraform apply" after
adding explicit source addresses in the configuration. Previously we just
left that implied under the assumption that everyone would run
"terraform apply" shortly after anyway, but there is a specific tricky
situation where the first change after upgrading is to remove a resource
from the configuration, leaving Terraform unable to complete the upgrade.
Because of that, we'll now explicitly direct users to run
"terraform apply" after upgrading. Along with that, there's a reminder to
make sure that "terraform plan" indicates no changes before upgrading, so
that completing the upgrade doesn't involve also applying changes to
remote objects.
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